How can a scientist predict how explosive the eruption of a volcano will likely be?

by using a cospec to check the gases
by using a tiltmeter to check the ground swelling
by studying the composition of its volcanic rock
by determining if the volcano is active, dormant, or extinct

By studying the composition of its volcanic rock

The amount of liquefied gas in the magma provides the driving force for explosive outbursts. The coherence of the magma, however, is also an essential factor in concluding whether an explosion will be critical or not.

An explosive eruption always occurs with some sort of blockage in the cavity of a volcano that restricts the discharge of gases confined in extremely viscous andesitic or rhyolitic magma. The high coherence of these forms of magma limits release of trapped gases. Therefore, such components can only be comprehended after studying the composition of its volcanic rock.